1. Field of the Invention
The present invention relates to an oscillation circuit for a proximity switch in which an amplitude of an oscillation varies in response to approach of an object to be sensed.
2. Related Art
It has been recently required to provide a proximy switch with a setting display function. With this function, a distance in which an object can be sensed with a high stability (e.g. a distance of about 80% of the maximum distance for sensing an object) is displayed. For the provision of the display function, it is necessary to integrally dispose in the proximity switch an oscillation circuit of which the oscillation amplitude varies depending on the distance from an object to be sensed to the switch.
In order to solve this problem, the inventors have devised a linear oscillation circuit, as shown in FIG. 3, as an example of the oscillation circuit which varies its oscillation amplitude in association with the distance of the object to be sensed. Although the circuit configuration of FIG. 3 does not belong to the prior art technology, for an easier understanding of the present invention, the linear oscillation circuit will be here described as a related art of the present invention.
The linear oscillation circuit comprises a constant current mirror circuit constituted with a constant current source CS.sub.0 to generate a current I.sub.0 and a current mirror circuit including transistors Tr4 and Tr5. Connected on an output side of the mirror circuit is a transistor Tr6 for an oscillation. The linear oscillation circuit further includes an LC oscillation circuit having a sense coil L and a capacitor C connected in parallel thereto, a bias current source CS.sub.1, and a diode connecting transistor Tr1 as a bias circuit connected between the current source CS.sub.1 and the LC resonance circuit. The transistor Tr1 has a collector linked to a base of the oscillation transistor Tr6. Furthermore, there is disposed a current mirror circuit for feeding back a current flowing through the transistor Tr6 to the parallel LC resonance circuit. The feedback current mirror circuit includes two transistors Tr2 and Tr3. When an upper end portion (an emitter Of the transistor Tr1) of the LC resonance circuit is set to a positive potential, the oscillation transistor Tr6 turns on to flow an output current I.sub.0 equal to the current from the constant current source CS.sub.0 through the transistor Tr6. The current I.sub.0 is fed back via the current mirror circuit including the transistors Tr2 and Tr3 to the LC resonance circuit.
The oscillation circuit is of constant current feedback type, namely, the constant current I.sub.0 is fed back to the LC resonance circuit. In consequence, the oscillation amplitude of this oscillation circuit, namely, a voltage V.sub.RP appearing across the LC resonance circuit is determined by the constant current I.sub.0 and conductance g of the resonance circuit as follows. EQU V.sub.RP =K.multidot.I.sub.0 /g (1)
where, K is a proportional constant. Moreover, since I.sub.0 and V.sub.RP of the expression (1) are values associated with an alternate current and hence may be regarded as effective values thereof.
Since the conductance value g varies depending on the distance l of an object Oj to be sensed, the oscillation amplitude V.sub.RP also varies in association with the distance l of an object Oj. This phenomenon is shown in FIG. 4. The oscillation voltage V.sub.RP is fed to a level discriminator (comparator) circuit so as to be discriminated by use of a predetermined level (having a hysteresis). As the object Oj approaches the sense coil L, the oscillation amplitude V.sub.RP decreases. When the amplitude V.sub.RP becomes equal to or less than the reference level, the proximity switch turns a sense signal on (sensing point, on point). A setting point is located at a position, as compared with the sensing point, slightly nearer to the sense coil L (e.g. 80% of the distance of the sensing or sense point as described above). Conversely, as the distance of the object Oj increases, the oscillation amplitude V.sub.RP becomes greater. Due to the hysteresis of the reference level, when the object Oj is at a position slightly further apart from the sensing point, the sense signal is turned off (off point).
Since the circuit of FIG. 3 is of a constant current feedback type, the change in the oscillation amplitude V.sub.RP depends only on the variation in the conductance of the LC resonance circuit. In consequence, when an LC resonance circuit having a small conductance variation is employed, it is impossible to attain a sufficient change in the oscillation amplitude (i.e. the graph of FIG. 4 has a small gradient) and hence the operation cannot be accomplished with a satisfactory stability.